1. Field of the Invention
The present invention relates to an end portion flow rate regulating apparatus for a paper-making machine headbox, in which wet paper stock is led from a tapered header through a tube bank to a slice chamber.
2. Description of the Prior Art
While computer paper is being stacked up after printing, sometimes the phenomena called "oblique tilt" occurs, wherein the paper layers stack up obliquely and slip a little one upon the other. Also, copy sheets would curl, and sometimes a sorter would stop due to problems with the paper. The principal causes of these troubles are considered to be fiber orientation angles of the paper or a difference of the fiber orientation angles between the respective sides of the paper. In general, in a machine-made paper sheet, many long and slender fibers are aligned in the paper making direction (MD). Therefore, the mechanical strength of the paper in the paper making direction (MD) is normally at least twice that in a cross direction (CD) of the machine extending at right angles to the paper making direction.
Although the direction of orientation of most of the fibers nearly coincides with the paper making direction, the orientation of the fibers of portions of the paper web made at sides of the machine (principal axis of fiber orientation) deviates slightly from the MD direction of the machine. This deviation is referred to as the "fiber orientation angle", and when this angle is large the paper is said to have a "poor fiber orientation property".
Although the fiber orientation angle can be determined from the measured tensile strength of a paper sheet, it is also possible to accurately measure the fiber orientation angle in a short period of time by using a measuring instrument called an SST (Sonic Sheet Tester). The SST makes use of the fact that normally the speed of a supersonic wave propagating through paper is high in the principal direction of alignment of the fibers.
If the direction of a jet ejected from a headbox includes a CD (cross-directional) component, then the fiber orientation deviates from the MD direction. If this deviation is great, a paper web having a poor fiber orientation is produced. Accordingly, a headbox which ejects jets having a small CD component is required.
In general, in order to eliminate the phenomena of "oblique tilt", the fiber orientation angle must be 2.degree. or less. Also, it is known that in a paper sheet having a little curl a difference between the fiber orientation angles at the front and rear sides of the paper sheet is small.
In this regard, U.S. Pat. No. 2,904,461 proposes means for injecting fluid from side plates (pond sides) of an air-cushioned headbox. Also, Japanese Patent Publication No. 43-12602 (1968) discloses bleed means for extracting liquid from side plates of an air-cushioned headbox. However, in these known apparatus, flow resistance at the sides of the air-cushioned headbox was increased due to a rectifying roll and the wall surfaces of the pond sides. As a result, the flow velocity is lowered just in front of a jet port of a slice. Hence, the flow directions of the jets vary, and (1) the basis weight of the paper at the sides of the wires becomes too low and (2) an uneven portion having a large basis weight is produced at locations just a little inside of the pond sides.
The above-described reference did disclose the concepts of enhancing the uniformity of the basis of the paper weight by injecting or extracting liquid to or from the sides of the headbox. In a subsequently developed hydraulic headbox not having a rectifying roll, sometimes there was a tendency to generate a similar uneven basis weight due to deficiencies associated with an end portion porosity of a tube bank and resistance offered by wall surfaces of the pond sides of the headbox. In addition, even if the basis weight were sufficiently uniform, if the paper making was effected under a ratio J/W (jet velocity to the wire velocity) of close to 1.00, then sometimes a component of the force on the stock in the CD direction became excessive and the fiber orientation angle became about 5.degree.-10.degree..
FIGS. 14 to 16 show one example of a hydraulic headbox in the prior art. Stock flows through a circular-to-rectangular transition approach pipe 1, then flows into a thin-tipped tapered header 2 having a rectangular section, and is divided into tubes 4 aligned in the widthwise direction of a tube bank 3. In addition, part of the stock in the header flows to recirculation piping 20. Reference numeral 19 designates a valve and numeral 21 designates a flow meter. An inlet portion 4' of each tube 4 has a small diameter in order to enhance the uniformity of the distribution of the flow rate in the widthwise direction, and the velocity of the flow is raised in this portion, whereby head loss increases.
Furthermore, an outlet portion 4" of each tube 4 has a larger cross-sectional area to lower the velocity of the flow for the purpose of preventing the velocity of the flow entering a slice chamber 5 from becoming excessive. The slice chamber 5 is divided in the vertical direction by sheet-like flow-suppressor elements 6 extending in the widthwise direction of the headbox. Thus, jets ejected from the tubes 4 are suppressed from mixing and establishing a large mass. Disturbance of the flow is limited by the flow-suppressor elements 6 to an extent corresponding to the intervals between the sheet-like elements. Also, since a shearing force is applied to the fluid, a state of dispersion of the fibers is greatly improved.
Accordingly, a smooth jet in which disturbance is small and the dispersion of fibers is excellent, is obtained. In addition, a top plate 7 and a bottom plate 8 of the slice chamber 5 converge in the direction of flow, and the top plate 7 can rotate about a pivot 10 to adjust the size of an ejection port 11 of the headbox. The size of the ejection port can be finely adjusted by flexing a lip 9 of the top plate 7 over the entire widthwise direction of the chambers. Reference numeral 18 designates piping for regulating the flow rate by bleeding wet paper stock from the pond sides. As the amount of stock so extracted increases, the more the jet begins to flow in the outward direction. During operation, the jets are preliminarily established to flow in inward directions by making the flow rates in the end tubes of the tube bank greater than the flow rates in the central tubes either by making the diameters of the end tubes larger than those of the central tubes or by providing end tubes at a pitch larger than that of the central tubes. The flow directions of the jets were regulated by appropriate bleeding of the stock through the piping 18.
FIG. 16 shows one example of the flow rate distributions in the widthwise direction of such a headbox. Although the orientation of the fibers presents no problem in ordinary paper, it was impossible to control the fiber orientation during operation. In addition, fluid was fed from inflow piping or recirculating piping in the heretofore known device for injecting fluid (wet paper stock) to the sides of a headbox. This resulted in a shortcoming in that if the recirculating flow rate is changed, then the flow rates at the end of the tube bank corresponding to the recirculating side would vary.
As described above, fluid is introduced into the end portion flow rate regulating apparatus in the prior art via inflow piping or recirculating piping. However, since these pipings are mounted at a location where the cross section of the flow path is changing, the flow rate of the fluid at that location is not identical to that in the header portion. On the basis of Bernoulli's theorem, therefore, the pressure (static pressure) of the fluid at such location is different from that in the header portion. In addition, the use of recirculating piping gave rise to the shortcoming that when the recirculating flow rate is regulated, the flow velocity change is comparatively greater, i.e. as compared to the use of inflow piping. Therefore, the pressure changes to a large extent. Furthermore, in the heretofore known method, although a stable fiber orientation can be obtained, it was impossible to widely control the fiber orientation during operation.